You can cancel white border around gallery this way.1. Turn on devoloper tool in Chrome2. Find element where border is, devoloper tool will show you whole way, where you can find code-Path looks like this: http://...your url..../templates/galerie-it is name of teplate/css/template.css ....here will be line number

So white border you will cancel in Joomla 2.5 template in template.css for example line number 1775

!! If your not succesful, try reset cache of browser too, if you are sure you erased right line with code, it helps sometimes !!